1. Field of the Invention
The present invention relates to centrifuge systems and more particularly, to a method of limiting the operating speed of a centrifuge rotor when an actual operating parameter value of the rotor is not within a predetermined range of an expected operating parameter value of the rotor.
2. Description of the Prior Art
A centrifuge instrument is a device by which liquid samples may be subjected to centrifugal forces. The sample is carried within a member known as a centrifuge rotor. The rotor is mounted to a rotatable drive shaft that is connected to a source of motive energy.
The centrifuge instrument may accept any one of a plurality of different centrifuge rotors depending upon the separation protocol being performed. Whatever rotor is being used, however, it is important to insure that the rotor does not attain an energy level that exceeds the capacity of the energy containment system of the instrument, or that exceeds a predetermined amount of centrifuge movement as a result of a rotor failure.
The energy containment and centrifuge movement reduction system(s) include all structural features of the centrifuge instrument that cooperate to confine within the instrument any fragments produced in the event of a rotor failure. These structural features include, for example, one (or more, concentric) guard ring(s), instrument chamber door and associated door latches. The energy containment system, however configured, has an energy containment threshold.
The total energy input to a system is equal to the sum of the energy dissipated in operation and the stored energy. Applied energy is stored by the rotation of the rotor. If the stored energy of a failed rotor exceeds the energy containment threshold of the instrument a fragment of the rotor may not be confined by the containment system. It is the stored energy that must be contained in the event of rotor failure.
The stored energy of motion, or the kinetic energy, of a rotor is directly related to its angular velocity, as specified by the relationship:
Kinetic Energy=xc2xd(Ixcfx892)
where I is the moment of inertia of the rotor, and where xcfx89 is its angular velocity.
Presently, the most direct manner of limiting rotor energy is to limit the velocity, i.e., the angular velocity or the speed, that the rotor is able to attain. It is also important to limit a rotor to its rated speed to insure its longevity, and the integrity of the samples, containers and centrifugation result.
One manner of rotor speed limitation is achieved by windage limiting the rotor. Windage limitation is a passive speed limitation technique. Windage limitation is the state of equilibrium between delivered motor torque and air friction losses of the rotor at a steady state speed.
Another way to limit rotor speed is to provide an overspeed control system in the instrument that affirmatively, or actively, limits the speed at which each given rotor is allowed to spin. For an active overspeed control system to limit rotor speed effectively it must typically ascertain the identity of the rotor mounted in the instrument.
Rotor identity information may be directly derived from the operator by requiring that the operator input identity information to the control system prior to the initiation of a centrifugation run. However, to protect against the possibility of an operator error, independent rotor identity arrangements are used. These rotor identity arrangements identify the rotor present on the drive shaft of the instrument and, based on this identification, permit the rotor to reach only a predetermined allowable speed.
Various forms of independent rotor identity arrangements are known. In one form each rotor in a rotor family carries a speed decal having bands or sectors of differing light reflectivity. A code is read by an associated sensor at a predetermined low angular velocity. This technique establishes an acceptable maximum rotor speed based on a rate of alternating light and dark pulses. In another form each rotor in the family carries a predetermined pattern of magnets. The magnets are sensed by a suitable detector, typically a Hall Effect device, to read the rotor code. U.S. Pat. No. 4,601,696 to Kamm is representative of this form of rotor identity arrangement.
Other arrangements for independent rotor identity sense a particular parameter of rotor construction in order to identify the rotor. In the arrangement disclosed in U.S. Pat. No. 5,037,371 to Romanauskas, the shape of a rotor mounted on the drive shaft is interrogated ultrasonically to generate a signal representative of the rotor""s identity. In U.S. Pat. No. 4,827,197 to Giebeler, the inertia of the rotor mounted on the shaft is detected and used as a basis for rotor identity.
Some overspeed protection systems limit operating speed based on a monitored operating parameter of a rotor rather than on the identity of the rotor. U.S. Pat. Nos. 5,600,076 and 5,650,578, both to Fleming et al., describe systems that monitor applied accelerating energy in order to ensure that the applied energy does not exceed the containment capability of the centrifuge chamber. The decision of whether to limit speed is made independent of the identity of the rotor, and it does not consider the expected behavior of the rotor.
There is a need for a method of overspeed protection that considers whether an actual operating parameter of a rotor is within a predetermined range of an expected value of the operating parameter of the rotor, and then limits the rotor speed based on the actual parameter.
The present invention is a method and system for limiting an operating speed of a centrifuge rotor. The method includes the steps of determining whether an actual parameter value of the rotor is within a predetermined range of an expected parameter value of the rotor, and limiting the operating speed when the actual parameter value is not within the predetermined range of the expected parameter value. At least one of the following determinations are made: (i) whether an actual energy required to accelerate the rotor from rest to a predetermined speed is within a predetermined range of an expected energy required to accelerate the rotor from rest to the predetermined speed, (ii) whether an actual change in energy required to accelerate the rotor from a first speed to a second speed is within a predetermined range of an expected change in energy required to accelerate the rotor from the first speed to the second speed, (iii) whether an actual energy loss due to windage of the rotor is within a predetermined range of an expected energy loss due to windage of the rotor, (iv) whether an actual time required to accelerate the rotor from a first speed to a second speed is within a predetermined range of an expected time required to accelerate the rotor from the first speed to the second speed, (v) whether an actual speed of the rotor is within a predetermined range of an expected speed of the rotor at a predetermined time, and (vi) whether an actual ratio of change in acceleration and difference of drag torque speed terms of the rotor is within an predetermined range of an expected ratio of change in acceleration and difference of drag torque speed terms.